Pulse oximetry systems generally include probes for attachment to an appendage of a patient for transmitting red and infrared signals to the patient's appendage and detecting the signals transmitted through or reflected from the patient's appendage. A monitor usually drives the probes and processes the probe signals to determine, among other things, the oxygenation of the patient's blood and the patient's pulse rate. Probe cabling typically interconnects the probe to a monitor to define the pulse oximetry system. Pulse oximetry probes are generally disposable or single-use products. By contrast, the monitors are intended for long-term use. Thus, many different probes are connected to a monitor over time.
In order to ensure functioning of the pulse oximetry system as intended, when a particular probe is interconnected, the monitor generally implements an initialization process. Initialization may involve a number of processes including a shorts and opens routine, probe family identification, qualification and calibration.
The shorts and opens routine is intended to identify potential malfunction conditions, such as short or open circuits, in the probe. In this regard, the probe typically includes circuitry for driving multiple LEDs—e.g., a red LED and an infrared LED—a detector circuit, and certain probe identification and calibration components, as will be described below.
In connection with one exemplary conventional shorts and opens routine, the voltage drop across two probe pins associated with the infrared LED is detected periodically. The measured voltage drop is compared to low and high threshold values to ensure that the measured voltage drop is within a proper operating range. In this manner, potentially dangerous probe conditions can be identified so as to better ensure patent safety. If the measured voltage drop is not within the proper operating range, the probe drive is disabled.
In connection with the probe family identification process, in the noted conventional oximeter, a signal applied across two probe pins may be used to determine whether a family identification diode is present. The presence or absence of this diode enables the monitor to set the probe family type, e.g., to determine whether the probe is a finger probe or an ear probe. This information is then used to operate the monitor. Both the shorts and opens analysis and the family identification analysis may be performed periodically, e.g., every two seconds during oximeter operation.
In addition to these processes, initialization may involve probe qualification and calibration. Qualification relates to verifying that the probe is a type of probe supported or authorized for use by the monitor and that the probe is otherwise functioning properly, i.e., that the probe may be used for patient monitoring or that full functionality may be implemented. This decision is binary in nature, i.e., involves a single determination as between two states. That is, the probe is either “qualified” for use in the system or “disqualified”. The result of a disqualification determination is generally that the probe drive components of the monitor are disabled to prevent any use of the probe in the system for patient monitoring, or certain functionality is disabled.
Calibration relates to adapting the system to the particular characteristics of the attached probe. In this regard, as noted above, each probe generally includes at least a red LED and an infrared LED. The center wavelengths of these LEDs vary from probe to probe. During a calibration process, the values of these wavelengths are identified. These values can then be used to customize the algorithms used for determining blood oxygen saturation, e.g., by appropriate selection of the coefficients of a series of algorithm terms. Calibration thus involves quantifying probe characteristics relative to an expected range of values.
More specifically, certain conventional systems interrogate a calibration resistor, Rcal, of the probe used to encode the wavelength values of the red and infrared LEDs During manufacturing, the wavelength values of these LEDs are determined. These values are encoded by the resistance value of the Rcal resistor which is wired across two pins of the probe. That is, each supported combination of red and infrared wavelengths is associated with an Rcal value and associated relationships are stored for use by the monitor processing unit. During initialization, a known potential or signal is applied across the Rcal pins and a value related to the Rcal resistor, such as a voltage drop, is determined. In turn, this value is used to select the proper coefficients for the corresponding red and infrared wavelengths.
It will be appreciated that proper functioning of a pulse oximeter depends on successful completion and accurate interpretation of these initialization processes.